Electrospinning is a technique known in the art for producing articles composed of polymer fibers. See U.S. Pat. No. 4,323,525. More recently, U.S. Pat. No. 7,276,271 disclosed a fibrous polymer tubular structure. Substrates with nanoscale aligned geometries are disclosed in U.S. Pat. No. 6,676,675. The use of electrospun fibrous polymer grafts in medical applications is disclosed in U.S. Pat. No. 7,172,765.
Modified electrospinning methods for forming directed fibers are also disclosed in U.S. Pat. No. 7,135,134. PCT Publication Nos. WO 2007/146261 and WO 2007/090102 disclose various aspects of polymer fibrous scaffold sheets.